An electrochemical capacitor, also known as a supercapacitor or an ultracapacitor, is an energy storage device that combines the high energy storage potential of a battery with the high energy transfer rate of a conventional capacitor. The performance characteristics of an electrochemical capacitor can be evaluated in terms of its specific energy, the amount of energy that can be stored per unit weight, and in terms of its specific power, the rate at which an amount of energy can be transferred in or out of that unit weight.
An electrochemical capacitor usually includes a hermetically sealed casing filled with electrolyte, a positive electrode and a negative electrode placed in the casing, a separator that separates the positive electrode space from the negative electrode space, and special lead terminals connecting the supercapacitor to external electric circuits.
One type of an electrochemical capacitor is an Electric Double Layer Capacitor (EDLC) that accumulates energy through the charging of an electric double layer at the electrode/electrolyte interface. One of the promising methods for improving the performance of EDLCs is the chemical modification of the positive electrode, for example, through immobilization of energy-accumulating polymers on its surface. Such electrochemical capacitors are called hybrid capacitors. As compared to an EDLC, a hybrid capacitor is characterized by a significantly higher specific energy and an increased operational voltage.
As mentioned above, immobilization of conductive polymers on the surface of the positive electrode of a supercapacitor can improve its performance. Conductive polymers are sub classified into two types [B. E. Conway, Electrochemical Supercapacitors, Kluwer Acad. Plen. Publ., NY, 1999, 698 p.]: (1) so called “organic metals” or conducting polymers—these are polymers with a conduction mechanism similar to that of metals and (2) redox polymers—i.e., compounds, in which electrons are transferred due to redox reactions between adjacent fragments of a polymer chain.
Examples of “organic metals” are poly(acetylene), poly(pyrrole), poly(thiophene), and poly(aniline). In partly oxidized form, these polymers are characterized by a very high conductivity and can be considered as salts consisting of positively charged “ions” of the polymer and charge-compensating counterions of the supporting electrolyte, which are uniformly distributed throughout the polymer structure and maintain the overall electric neutrality of the system. In solid state physics, the cation-radical, which is partly delocalized throughout the polymer fragment, is called a polaron. The polaron theory of conductivity is acknowledged to be the main model of charge transfer in conducting polymers. [Charge Transfer in Polymeric Systems, Faraday Discussions of the Chemical Society. 1989. V.88].
“Organic metals” may be obtained using a method of electrochemical oxidation of appropriate monomers on the surface of an inert electrode. These polymers may be converted from the conducting (oxidized) state to non-conductive (neutral) state through the variation of the electrode potential. The transformation of the polymer from an oxidized state into a neutral state is accompanied by the release of charge-compensating counterions from the polymer to electrolyte solution, in which the process develops, and vice versa.
Both purely organic systems and polymer metal complexes refer to redox polymers [H. G. Cassidy and K. A. Kun. Oxidation Reduction Polymers (Redox Polymers), Wiley-Interscience, New York, 1965].
Polymer metal complexes may be obtained through electrochemical polymerization of initial metal complexes. Examples of redox polymers are polypiridine complexes poly-[M(v-bpy)x(L)y], where:    M=Co, Fe, Ru, Os;    v-bpy=4-vinyl-4′-methyl-2,2′-bipyridine;    L=v-bpy (4-vinyl-4′-methyl-2,2′-bipyridine), phenanthroline-5,6-dione, 4-methyl-phenanthroline, 5-aminophenanthroline, 5-chlorophenanthroline;    x+y=3 [Hurrel H. C., Abruna H. D. Redox Conduction in Electropolymerized Films of Transition Metal Complexes of Os, Ru, Fe, and Co, Inorganic Chemistry. 1990. V.29. P.736-741].
Redox centers, i.e. atoms participating in redox reactions, in the polymer are metal ions that should have different oxidation states. Thus, complexes of metals, which have only one possible oxidation state (for example, zinc, cadmium) will not form redox polymers. In order for a redox polymers to be conductive, a highly developed system of conjugated π-bonds in a ligand environment must be present, with these π-bonds functioning as conductive “bridges” between the redox centers. When a redox polymer is completely oxidized or completely reduced (i.e. all its redox centers are in one identical oxidation state), the charge transfer along the polymer chain is impossible and the conductivity of a redox polymer is close to zero.
When redox centers have different oxidation states, electron exchange between redox centers is possible. In this case, the electric conductivity of redox polymers is proportional to the rate constant of the electron self-exchange reaction between redox centers (kse) and concentrations of oxidized ([Ox]) and reduced ([Red]) centers and in a polymer, i.e. conductivity of redox polymer ˜kse[Ox] [Red].
As compared to electrodes modified by “organic metals” (conducting polymers), redox polymers and electrodes with redox polymers on their surface (i.e. electrodes modified by redox polymers) potentially offer higher specific energy owing to the greater contribution of the Faraday component of capacity to the overall capacity of the polymer, which is associated with multi-electron oxidation/reduction of metal centers.
The traditional method of manufacturing hybrid capacitors equipped with positive electrodes modified by redox polymers includes the following stages.
Manufacturing of electrodes. Electrodes may consist of a porous electrically conductive material (e.g. substrate) and current collectors with high electronic conductivity.
Deposition of the polymer on the positive electrode. The polymer may be deposited on the positive electrode via electrochemical polymerization of metal complexes on the surface of a porous electrically conductive electrode substrate of the positive electrode in an electrolysis bath. The negative electrode in the electrolysis bath may be an electrochemically inert material, for example, carbon cloth, that performs the function of an auxiliary electrode. The electrolysis bath is filled with an electrolyte, which may be a solution consisting of organic solvent, a metal complex, and a substance soluble in this solvent to a concentration of no less than 0.01 mol/L and containing ions that are electrochemically inactive within the range of potentials between −3.0 V to +1.5 V (from here on the values of potentials are given versus a standard silver/silver chloride reference electrode). To perform polymerization using an external electric power source, constant voltage or pulses of voltage are supplied to the electrodes in the electrolysis bath. The duration of polymerization process may range from 10 hours to 24 hours;
Assembly of a capacitor. The assembly of the hybrid capacitor includes the placement of the positive electrode modified by the polymer, the negative electrode, and a separator, which separates the electrodes, in a casing. The hybrid capacitor is then filled with an electrolyte solution consisting of an organic solvent and a substance soluble in this solvent to a concentration of no less than 0.01 mol/L and containing ions that are electrochemically inactive within the range of potentials between −3.0 V to +1.5 V. After the hybrid capacitor is filled with the electrolyte, the casing of the hybrid capacitor is hermetically sealed.
Conditioning of a capacitor. Conditioning the capacitor implies charging and discharging of the capacitor several times. The repeated charging and discharging removes any impurities from the electrolyte and either of the electrodes due to electrochemical oxidation and reduction of the impurities. The duration of capacitor conditioning may be no less than about 50 hours.
The main disadvantage of the traditional method for manufacturing of electrochemical capacitors equipped with positive electrodes modified by redox polymers is the presence of a special stage for the deposition of the polymer onto the porous substrate of the positive electrode prior to the assembly of the capacitor. This stage requires both additional equipment for polymerization and significant time for the process to occur. Additionally, during the assembly of the capacitor, in particular, during the arrangement of electrodes in the casing, the polymer layer may be damaged, which may result in the degrading of electrochemical properties of the product as a whole.
Therefore, a need exists to create a redox polymer modified positive electrode without the use of special equipment and without the risk of damaging the redox polymer immobilized on the surface of the positive electrode.